Oh, ok you're back. Right. First thing, you need to see this video of a giant spider crab moulting (hat tip PZ)

Once you've seen it and read PZ's piece on moulting, or ecdysis. The linked video of a spider crab moulting in PZ's piece is the one featured here. Very instructive, if not a little creepy. But remember that trilobites - being proper arthropods - exited the old body exoskeleton old school i.e. head first, not arse first like a common chelicerate.

Ok, now we have an idea of what ecdysis is, this Palaeoporn is an example of what happens when things go wrong . . . very wrong.

The photo at the top is of the trilobite Redlichia takooensis, or rather what's left of one after a particularly difficult mould. Just to remind you, at right is what a Redlichia takooensis should look like. R takooensis is a Lower Cambrian trilobite with a 15 segment thorax and small pygidium or tail. A major characteristic of R takooensis are the large spines, on either side of the head, at the back of the head in the centre, and two large spines on the axial rings of segments 6 and 11 (counting back from the head) (What? Axial ring. The middle part of the thorax. What? Thorax. The middle part of the animal between the head and the back end (pygidium). What? Yes the pygidium is very small, it's a micropygous trilobite. What? No. The division head-thorax-pygidium is not how the trilobite got it's name. The "three lobes" refers to the three zones of the thorax, the plura on the left, the axial ring, and the plura on the right).

There are probably spines on each axial ring, but 6 and 11 have the biggest. The spine coming off the rear of the trilobite, seen in the photo (above right), is from segment 11 - so the spines were pretty long.

To give an idea of what R. takooensis looked like I'm going to cheat an use another trilobite to approximate Redlichia.

Notice the spines. These will be very important in what happens in extreme ecdysis. In life, the spines would have not sat flat along the thorax, but would have been mobile and could be moved in an arc, from flat along the body (as above and in complete trilobite photo above right), through some acute angle (as seen in the ParaRedlichia diagram above).

What you don't see is the series on notches on the back margin of the first 5 segments of the thorax. These would have acted to lock the first 6 segments in place making this portion of the thorax act as a rigid body. That's also important.

What's also important is that the trilobite could bend. It could flex into an arc - though not into a ball like later trilobites (an important feature for Anomalocaris as discussed previously). Also the maximum flexure occured around segment 5.

OK, ecdysis in Redlichia. Normally Redlichia would have operated in a similar way to Paradoxides shown below.

Maneuvers during exuviation of Paradoxides 2. Left side view, thick line on head represents the suture line. 3. Flexure of body. 4. Front view showing open sutures and animal beginning to emerge (stippled). 5. Side view of animal beginning to emerge. (Whittington 1990).

The trilobite arched upward, planted the front of the head in the sediment and pushed using the back end as a brace. This put pressure on the head and the sutures around it. These part, with the cheeks on either side parting from the central area of the head, and the front part of the head separating and lifting upwards. This allowed the soft new body to emerge through the front of the head and 'head' off, leaving the cast of exoskeleton behind.

There is one difference (alright two). The spines. These could have been brought into play if the old exoskeleton didn't play ball, as shown below.

Here, the rear spine could have been used as a lever to apply more pressure on the head sutures, forcing them to separate and allow the trilobite to exit. How do we know this? well, a) it seems like a reasonable thing to do, and b) we have the specimen figured as Palaeoporn 16 (and others).

Let's remind ourselves of the specimen, 'cos its been a while.

What we have here is one disarticulated trilobite. There is a split in the thorax between segments 4 and 5, and another between segments 10 and 11. The thorax has been split into three pieces, comprising segments 1-4, 5-10, and 11-15 (plus pygidium).

Which brings us to a very important question. Where's the head?

The left free cheek is there, roughly where it should be at the front end, but it's inverted. It's inside out. Flipped over. The nice curved margin you can see is actually where it joined the eye. But the rest of the head?

Well, most is missing, but see the white and orange line running away from the free cheek and pointing to the 2 o'clock position? That's what's left of the head. It's the lower part of the head, basically a thin band of exoskeleton (called the doublure) which runs around the underside of the head and attached to both free cheeks and the front of the central portion of the head (the doublure is show in black at right). That central top portion of the head is gone, but the doublure is rammed into the sediment and twisted.

To give you some idea, here is a somewhat stylised diagram to illustrate.

What we have is one very sad and sorry trilobite. But, knowing what we know of ecdysis in Redlichia we can piece (Ha!) together what happened.

Knowing that the first 6 segments could lock together, and and the spine on segment 11 was used as a pivot point, simple mechanics tells us the the maximum forces exerted at or around segment 5, (where the maximum curve would be - the locking of segments 1-6 would re-enforce this), and around segment 11.

So, the trilobite arched up, rammed the front of the head into the sediment and braced with the spine on segment 11. Nothing happened.

Eventually the arching and the bracing built up so much pressure that something had to give. In very quick succession, the exoskeleton behind the head went (we know this can happen from Palaeoporn 15), as did the sutures on the head. But also the pressure acting on segment 5 became too much and the exoskeleton ruptured between segments 4 and 5. Also to pressure around segment 11, caused by bracing against the spine, causes the exoskeleton to rupture between segments 10 and 11.

The trilobite exited the broken exoskeleton, probably taking the middle portion of the head with it. That left the free cheeks separated from the central head exoskeleton, but still attached to the doublure, which was buried in the sediment. The left free cheek, still attached on its outer side, rotated outward to lie inside out, while the other is buried.

Tough moult!

But there's more. Here's another example. Similar story with similar results (sorry it's in black and white. I could say it's in black and white for the ambiance, but I just don't have a colour photo of it).

This time the thorax exoskeleton gave way behind segment 6 and behind segment 11. And this time we have the head. And both free cheeks, but they are inverted an displaced slightly. Also the doublure is present. it's the thick band right at the front of the head.

This is a classic example of a broken, incomplete, trilobite providing more information than a perfect specimen would. They provide information on trilobite activity and mechanical information on the exoskeleton.

One obvious question... did the trilobite actually escape and survive? Given that the disarticulated pieces are in close proximity it begs the question whether what we are seeing is a molt gone wrong (as you describe), but the animal remaining trapped in the old molt skin and dying, thus keeping most of the exoskeleton fragments in close proximity to each other even though they are no longer (or weakly) attached to each other. In my own experience rearing insects, its hard (in fact, I would say impossible without assistance) for an insect which has a problem during molting to 'rescue' itself from the remains of its molt skin. The trilobite molt skin is probably more rigid, but I would still think it would be very difficult to extricate itself once it snapped in the middle.

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Palaeontologist, interrupted. In a previous life I worked on Ediacaran and Early Cambrian palaeontology, palaeoecology and taphonomy. While doing all that I also discovered talk.origins . . . the rest was history. I subsequently moved on to a real job, mainly because they pay me. And for the lawyers, this blog represents my opinions only and not those of my employer.